The conventional technique utilized in the commercial production of sodium bicarbonate is the solution process. In the solution process, soda ash is dissolved in spent reaction liquor from prior reaction, consisting of water and small quantities of dissolved soda ash and sodium bicarbonate. The solution is then carbonated by sparging CO2 to the bottom of large carbonate columns to precipitate crystals of sodium bicarbonate. Carbonation of sodium bicarbonate is an example of a gas-liquid-solid reaction. The sodium bicarbonate crystals are typically separated from the liquor using centrifuges and dried to yield highly purified, high density crystals. Disadvantages of the conventional method are that the procedure requires several steps, and necessitates the use of separation equipment, drying of the product, and the handling of large volumes of liquids.
It has also been proposed to make sodium bicarbonate by various “dry carbonation” techniques. In U.S. Pat. No. 276,990 (Carey et al.) and U.S. Pat. No. 574,089 (Hawliczek), a sodium bicarbonate product is formed by placing hydrated soda ash in a revolving cylinder and then introducing carbon dioxide into the cylinder. In both patents, reaction times are of the order of five to six hours.
U.S. Pat. No. 3,647,365 (Saeman) teaches a process in which hollow sodium bicarbonate beads of low density are prepared in a multistage reactor from hydrated soda ash, small amounts of water and carbon dioxide. This process requires several steps and must proceed slowly, with carbonation times exceeding one hour and drying times up to eight hours. The soda ash must first be hydrated in a separate step, and the reaction must occur at a temperature above 95.7° F. to produce commercially acceptable reaction rates.
More recently, Krieg et al. (U.S. Pat. No. 4,459,272), (owned by the assignee of the present invention) described a process for the preparation of sodium bicarbonate by the reaction of a solid, particulate sodium carbonate-containing material with liquid water in a carbon dioxide-rich atmosphere. In the Krieg process, the particulate mass is mixed with the water and carbon dioxide in an internally agitated or externally rotated or vibrated reactor. The reaction is carried out at temperatures of from 125° F. to 240° F. under atmospheres containing from 20% to 90% carbon dioxide by volume. Using Krieg's terminology, the “dry carbonation” process is carried out under reduced water vapor partial pressures to promote evaporation of water from the surfaces of the reacting carbonate particles, and to maintain high carbon dioxide partial pressures in the reactor atmosphere. Products formed by the process have apparent bulk densities as high as 50-60 lb/ft3.
On the other hand, Sarapata, et al. in U.S. Pat. No. 4,664,893 (also owned by the assignee of the present invention) disclose that in the dry carbonation of sodium carbonate, it is necessary to react a substantially saturated feed gas stream (relative humidity in excess of 90%) to maintain adequate reaction rates.
Kurtz, et al. in U.S. Pat. No. 4,919,910 (also owned by the assignee of the present invention) disclose a process for the dry carbonation of potassium carbonate, which comprises reacting dry potassium carbonate, carbon dioxide and water vapor at atmospheric pressure and under turbulent mixing conditions to produce potassium bicarbonate.
WO 93/11070, published Jun. 10, 1993 and issued to Falotico and owned by the assignee of the present invention, a process is provided for the dry carbonation of Trona, which comprises:                (a) passing Trona particles through a reaction zone (e.g., an internally agitated or externally rotated or vibrated reactor);        (b) introducing into the reaction zone a gas stream containing from about 12% to 100% carbon dioxide by volume, any remaining percentage of the gas stream being an inert gas such as air or nitrogen, the gas stream being heated to a temperature within the range of about 140° F. to about 160° F. [about 60° to about 71.1° C.], preferably about 150° F. to about 155° F. [about 65.6° to about 68.3° C.];        (c) initiating the reaction by introducing water into the reaction zone to form a gas mixture of water vapor and the gas stream from step (b), so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream from step (b);        (d) thereafter during the course of the reaction, intermittently introducing water into the reaction zone to form “a gas mixture of water vapor and the gas stream from step (b), so that the water vapor content of the gas mixture reaches essentially 100% of saturation at the temperature of the gas stream from step (b), if required to maintain the reaction with a gas stream containing less than 100% CO2;        (e) maintaining the gas mixture or gas stream in direct contact with the Trona particles during the reaction and continuing the reaction with production of water from the Trona and no external source of water when the gas stream is close to 100% CO2, until the sodium carbonate in the Trona particles is essentially all converted to sodium bicarbonate with a water content of less than about 4 percent by weight;        (g) discharging the gas stream or gas mixture from the reaction zone after contact with the particles, and        (h) discharging from the reaction zone reacted particles having a lower bulk density, a higher specific surface area and consequently higher absorption property than the Trona feed, and a water content of less than 4 percent by weight and different amounts of impurities (up to 20%) depending on their presence in the starting Trona ore, wherein sodium bicarbonate product formed by the dry carbonation has a surface area of about 0.3 m2/g. In contrast “wet” produced sodium bicarbonate has a surface area from about 0.05 to 0.09 m2/g.        
Unfortunately, the process as proposed in WO 93/11070 requires at least 3 hours to convert the Trona to at least 98% sodium bicarbonate during a continuous reaction.
Prior art “dry carbonation” techniques are subject to particular disadvantages. In some of these processes, the carbon dioxide concentration must be high and the reaction temperature must also be high, or the reaction rate is prohibitively low. In some, the product must be dried. Despite a passing reference to the use of calcined Trona in U.S. Pat. No. 4,459,272, none of the patents disclose the surprising benefits and properties that result from the boundary layer carbonation process of the present invention.
Sodium bicarbonate has also been produced, as well as utilized, in dry sorbent injection processes for removing sulfur dioxide emissions from the combustion gases of fossil fuel-fired burners. Such techniques have commanded considerable attention recently, particularly because they present the lowest “first cost” alternative for removing potentially dangerous sulfur dioxide and other gases from flue gases. Sodium bicarbonate has been demonstrated to be a very effective sorbent in the dry sorbent injection process. However, the cost of pharmaceutical and food grade sodium bicarbonate, as currently produced, is a major drawback to its use for such purpose.
U.S. Pat. No. 3,846,535 (Fonseca) and U.S. Pat. No. 4,385,039 (Lowell et al.) disclose methods for regenerating sodium bicarbonate from sulfate-containing solid waste formed by dry sorbent injection with sodium bicarbonate. The Fonseca regeneration step is carried out by forming an aqueous solution of the sodium sulfate-containing waste, and treating such solution with ammonium bicarbonate to precipitate sodium bicarbonate. The sodium bicarbonate is then separated, dried and recycled for further use. Lowell et al. disclose a regeneration step which involves dissolving the solid desulfurization reaction product in an alkaline liquor, which contains borate ions and/or ammonia. Carbonation of this liquor results in a sodium bicarbonate precipitate. The Fonseca and Lowell et al. processes thus both suffer from the use of complicated and capital intensive solution operations.
Sarapata, et al. in U.S. Pat. No. 4,664,893, mentioned above, also disclose that their “dry carbonation” process may be used to desulfurize flue gas streams, wherein the flue gas is contacted with a solid alkali metal or ammonium bicarbonate containing sorbent to react with sulfur dioxide in the flue gas. The resulting solid waste is separated and removed from the gas stream. The cleansed gas stream, from which the solid waste has been removed, is cooled; the gas stream is saturated with water vapor; and the gas stream is thoroughly mixed with a particulate alkali metal or ammonium carbonate. The bicarbonate produced thereby is then utilized to contact the hot flue gas for further desulfurization thereof.